Always run pytest tests in parallel

[refi64]

Ref. #1673.

@gnprice This one was pretty easy. ;)

[gvanrossum]

Are you aware it failed? :-)

[refi64]

@gvanrossum My bad. Forgot to add pytest-xdist as a dependency...

[refi64]

Looking at the build log, this seems to have significantly cut down the pytest time from ~80-90 seconds down to ~40, at least on Python 3.5.